What Does It Mean That the World Is Blocked by an Abyss?

Ning Yuan, who was reincarnated into another world, thought this was just an ordinary martial arts world. But an accident allowed him to meet a real strong man, and he learned that this world had been ravaged by the abyss. Even now, the demon lords of the pseudo-abyss are still prying into the world wantonly, and their minions are always trying to eat the entire world.